1. Field of the Invention
The present invention relates to a Group III nitride semiconductor light-emitting device exhibiting improved emission output.
2. Background Art
Conventionally, the following Group III nitride semiconductor light-emitting devices have been known. Japanese Patent Application Laid-Open (kokai) No. 2002-299762 discloses a laser in which an n-type cladding layer and a p-type cladding layer, each layer having a two-layer structure, are disposed respectively at the top and bottom of the core comprising an active layer and guide layers enclosing the active layer. In this laser, free carrier absorption loss is reduced by lowering the impurity concentrations of the n-type cladding layer and the p-type cladding layer closer to the active layer respectively than those of the n-type cladding layer and the p-type cladding layer farther from the active layer, to thereby reduce the threshold voltage. Each n-type cladding layer of the two-layer structure is a superlattice layer.
Japanese Patent No. 3909694 discloses a Group III nitride semiconductor light-emitting device comprising an n-type contact layer formed of n-type GaN, an n-type cladding layer formed of n-type AlGaInN, and an active layer, wherein a crack prevention layer formed of n-type GaN is provided between the n-type cladding layer and the n-type contact layer. The impurity concentration of the crack prevention layer is smaller than that of the n-type contact layer. By virtue of crack prevention layer formed of GaN, the increase of waveguide loss is suppressed.
Japanese Patent Application Laid-Open (kokai) No. 1998-4210 discloses a light-emitting device wherein a buffer layer, an n-type layer (first low-impurity concentration layer) formed of Si-doped GaN (thickness: 0.6 μm, Si concentration: 2×1018/cm3, and electron concentration: 1×1018/cm3), a high-carrier concentration n+-type layer (high-impurity concentration layer) formed of Si-doped GaN (thickness: 4.0 μm, Si concentration: 4×1018/cm3, and electron concentration: 2×1018/cm3), an n-type layer (second low-impurity concentration layer) formed of Si-doped GaN (thickness: 0.5 μm, Si concentration: 1×1018/cm3, and electron concentration: 5×1017/cm3), and a light-emitting layer are disposed in this order.
Japanese Patent Application Laid-Open (kokai) No. 2001-196702 discloses a laser diode wherein an n-type contact layer, an n-type cladding layer formed of Si-doped Al0.08Ga0.92N (thickness: 1 μm, and electron concentration: 5×1017/cm3), an n-type guide layer formed of Si-doped GaN (thickness: 100 nm, and electron concentration: 5×1017/cm3) and an active layer are provided in this order on.
Japanese Patent Application Laid-Open (kokai) No. 2001-44497 discloses a light-emitting device wherein a buffer layer, a contact layer formed of Si-doped AlGaN (thickness: 3 μm, and Si concentration: 1×1018/cm3), a cladding layer formed of Si-doped AlGaN (thickness: 1 μm, and Si concentration: 1×1018/cm3), an optical waveguide layer formed of Si-dope GaN (thickness: 0.1 μm, and Si concentration: 1×1018/cm3), and a light-emitting layer are disposed in this order.
The above patent documents disclose a light-emitting device in which a Group III nitride semiconductor was hetero-epitaxially grown on a heterogeneous substrate such as sapphire through MOCVD. In such light-emitting device, the n-type layer to bond with the light-emitting layer, has a low Si concentration of 5×1018/cm3 or less (including no impurity added) so as not to deteriorate the crystallinity of the light-emitting layer.
However, the present inventors have found that in the light-emitting device in which a Group III nitride semiconductor was homo-epitaxially grown on a Group III nitride semiconductor growth substrate, the emission output does not increase compared to the light-emitting device employing a sapphire substrate when the n-type layer to bond with the light-emitting layer has a low Si concentration of 5×1018/cm3 or less.
The present inventors assumed the reason of this as follows. When a Group III nitride semiconductor is hetero-epitaxially grown on a heterogenous substrate such as sapphire through MOCVD, high-density pits are enlarged in diameter from lower layer to the upper layer along with growth as shown in FIG. 7, due to a large lattice mismatch between the substrate and the growing Group III nitride semiconductor. The pit diameter was large, 150 nm to 200 nm and the pit density was large, 1×108/cm2 to 1×109/cm2 on the top surface of the light-emitting layer. Since the pits are filled with a p-type layer semiconductor, the distance is equivalently shortened between the n-type layer and the p-type layer to which a voltage is applied. Therefore, electrons and holes are easily recombined in the light-emitting layer even if the n-type layer to bond with the light-emitting layer has a low Si concentration of 5×1018/cm3 or less and a low conductivity. As a result, a higher emission output is obtained.
On the contrary, when a Group III nitride semiconductor is homo-epitaxially grown on a Group III nitride semiconductor growth substrate, the crystallinity of the epitaxial grown layer is improved, the pit density is low and the pit diameter is small in the light-emitting layer. When the n-type layer to bond with the light-emitting layer has a low Si concentration of 5×1018/cm3 similarly as in the light-emitting device employing a sapphire substrate, the effect of the p-type layer to fill in the above pits is reduced, the equivalent distance is lengthened between the p-type layer and the n-type layer to which a voltage is applied. Therefore, the present inventors thought that the probability of recombination of electrons and holes in the light-emitting layer is reduced, and a higher emission output cannot be obtained compared to the light-emitting device employing a sapphire substrate.